refactor: consolidate StorageOwnership to the generated StrEnum (#735)

## Summary

- Removes the duplicate `StorageOwnership` `Literal` from `_types.py`.
- Reuses the auto-generated `StorageOwnership` `StrEnum` from
`_models.py` in the dataset, key-value store, and request queue
collection clients.
- Updates the unit tests to pass enum members
(`StorageOwnership.OWNED_BY_ME` / `SHARED_WITH_ME`) instead of raw
strings.

Closes #700.